Lesson 1/Learning Event 1
Compare Figures 3 and 4. Using the Sieve Analysis Data Form it is easy to calculate the percentages of
the major soil materials: gravels, sand, and fines. Cobbles are not included in the analysis. Figure 4
shows the particle distribution curve of a predominantly sandy soil. Figure 5 shows distribution curves
for two well-graded and two poorly-graded soil types. "G" stands for gravel, "S" stands for sand.
Therefore, a GW is a soil which is well-graded and predominantly gravel.
Well-graded soils (GW and SW) are separated by a long curve spanning a large size range with a
constant or gently varying slope. Uniformly graded soils (SP curve) would be represented by a steeply
doping curve spanning a narrow size range, and the curve for a gap-graded soil (GP curve) will flatten
out in the area of the grain size deficiency.
You may also calculate the percentage of each grain type using the Grain Size Distribution Graph. Note
that the abscissa (the horizontal coordinate of a point) of the graph is divided into percent by weight. To
calculate the percent of a given grain type (for example, sand) begin by tracing a line straight down from
the sieve size that retains gravel, #4. Where that line intersects the distribution curve, trace a line
horizontally to the left and note the percent value where that line intersects the abscissa. Next trace a
line directly down from the #200 sieve size at the top. Where that line intersects the curve, make a line
horizontally to the left again. Subtract the second percent value from the first. The difference equals the
percentage of sand in the soil sample.
The rule stated earlier, the higher the density the stronger the soil, applies to grain size. Generally,
coarse grain soils can be compacted to a greater density than fine grained soils. Frequently, the largest
practical size for use in a construction project is one-half the compacted thickness. (Refer to Table 1.)
Note that fines can be silts, clays, or a combination of both.
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